A spark plug is used for igniting a fuel gas in an internal combustion engine. In such a spark plug, a gap for generating spark discharge for ignition (also called “spark gap”) is provided between a center electrode and a ground electrode. In general, the ground electrode is welded to a forward end portion of a metallic shell. In some cases, in order to enhance heat resistance, the ground electrode has a multilayer structure in which outer and inner portions of the ground electrode are formed of materials that differ in thermal conductivity and hardness (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2012-99496).
The welding interface between the metallic shell and the multilayer ground electrode is formed by the constituent material of the metallic shell and the various materials forming the ground electrode. Therefore, in the case where the inner portion of the ground electrode is formed of a material of low hardness such as copper (Cu), the material of low hardness may lower the strength of welding between the metallic shell and the ground electrode. Also, in some cases, the shape of the juncture portion of the ground electrode may be a cause of lowering the strength of the ground electrode. As described above, there yet remains room for enhancement of the reliability of joining between the metallic shell and the ground electrode.
The present invention has been accomplished so as to address at least the above-described problem, and the present invention can be embodied as the following modes.